The invention relates to a method and apparatus for reducing electrolytic interference with metal structures provided with external insulating coating and embedded in electrolyte in a foreign direct-current field produced by a cathodic protection electric circuit or by dynamic stray currents.
When facilities for cathodic protection of buried steel structures are in operation, a protective current flows from an auxiliary anode through the ground to the surface of the structure being protected which constitutes a cathode. When other, foreign metal structures are situated in the protective current path, a part of said current enters such structures and is discharged into the ground at another point to continue its flow to the cathode. As regards the foreign structures, the said part of the protective current acts as stray current. It uses the foreign structures, such as pipelines and cables, as electric conductors and at points where it leaves the structure to the ground it causes the structure to corrode. In principle, it is unimportant whether the current results from the operation of cathodic protection systems (with external current supply or with galvanic anodes) or whether it comprises dynamic stray currents which flow from one line structure to another and the source of which are, in the first place, d.c.-electrified railroad facilities with negative pole earthed. The magnitude of the current drained from the structure to ground per unit area of the structure is a measure of significance of the corrosion process.
The lengthwise flow of the interfering current to remote areas where it leaves the foreign structures to the ground usually requires detailed measurements over extensive sections of the line structure to find the critical spot, i.e., the geographic point of the most adverse effect of the electrolytic phenomenon, which corresponds with the point where the greatest part of the interfering (stray) current is discharged from the foreign metal line structure.
When the existence of interference has been detected and when the point of the most adverse effect of interference has been found, the next step is to design provisions for eliminating or mitigating the interference. In principle, these provisions consist of preventing uncontrolled discharge of the interfering current to the ground through holidays in the coating of the foreign structure.
The widely used method of interference bonding always has a certain favorable effect on the foreign structure exposed to interference and is always accompanied by a certain adverse effect on the interfering, cathodically protected structure. The degree of favorable or adverse effect is determined by a number of factors such as, for instance, the distance of the auxiliary anode (groundbed) from the foreign structure, the rectifier voltage and current and the transition (coating) resistance of the two structures. In extreme cases of interference, the current required for the interference bond may amount to as much as 40% or even 70% of the output of the cathodic protection installation to provide cathodic protection for the remote sections of the foreign line structure when its protective coating is poor or its surface bare. This necessitates raising the output voltage of the rectifier by as much as 200% if the same level of cathodic protection is to be provided for the interfering structure as in the absence of the interference bond.